Direct epoxidation of higher olefins (containing three or more carbons) such as propylene with oxygen and hydrogen has been the focus of recent efforts. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing carrier (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium and a titanium zeolite (see, e.g., JP 4-352771; U.S. Pat. Nos. 6,008,388, 6,498,259). Unfortunately, these catalyst systems also produce alkanes from the corresponding olefins. U.S. Pat. No. 6,005,123 discloses that a sulfur compound may be used to inhibit propane formation in the epoxidation of propylene, but it also inhibits the epoxides formation. There is a continued need to find new catalysts that inhibit hydrogenation of olefin without decreasing its epoxidation activity.
Thiol-functionalized mesoporous silicas are reported to selectively adsorb Pt2+ and Pd2+ (see Ind. Eng. Chem. Res. 43 (2004) 1478). On the other hand, the hydrogenation activity of Pt or Pd is strongly inhibited by the thiol functionality (see J. Chem. Eng. of Japan 35(3) (2002) 255).